Thu·Dec 10, 2020Thursday·December 10, 2020

1. 01*

   Evidence for the reduction of nuclear level density away from the $\beta$-stability line

   Pratap Roy · K. Banerjee🇮🇳 · T. K. Rana · S. Kundu · S. Manna · A. Sen · D. Mondal · J. Sadhukhan🇮🇳 · M. T. Senthil Kannan · T. K. Ghosh🇮🇳 · S. Mukhopadhyay🇮🇳 · Deepak Pandit🇮🇳 and 5 other authors

   The isospin dependence of nuclear level density has been investigated by analyzing the spectra of evaporated neutrons from excited ${}^{116}$Sn and ${}^{116}$Te nuclei. These nuclei are populated via $p$ + ${}^{115}$In and ${}^4$He + ${}^{112}$Sn reactions in the excitation energy range of 18 - 26 MeV. Because of low excitation energy, the neutron spectra are predominantly contributed by the first-chance decay leading to the $\beta$-stable ${}^{115}$Sn and neutron-deficient ${}^{115}$Te as residues for the two cases. Theoretical analysis of the experimental spectra have been performed within the Hauser-Feshbach formalism by employing different models of the level density parameter. It is observed that the data could only be explained by the level density parameter that decreases monotonically when the proton number deviates from the $\beta$-stable value. This is also confirmed by performing a microscopic shell-model calculation with the Wood-Saxon mean field. The results have strong implication on the estimation of the level density of unstable nuclei, and calculation of astrophysical reaction rates relevant to $r$- and $rp$-processes.

   nucl-exnucl-thPRC·14 citations
2. 02*

   Low-spin particle/hole-core excitations in ${}^{41,47,49}$Ca isotopes studied by cold-neutron capture reactions

   S. Bottoni🇮🇹 · N. Cieplicka-Oryńczak🇵🇱 · S. Leoni🇮🇹 · B. Fornal🇵🇱 · G. Colò🇮🇹 · P.F. Bortignon🇮🇹 · G. Bocchi🇮🇹 · D. Bazzacco🇮🇹 · G. Benzoni🇮🇹 · A. Blanc🇫🇷 · A. Bracco🇮🇹 · S. Ceruti🇮🇹 and 18 other authors

   We present recent results on the structure of the one-valence-particle ${}^{41}$Ca and ${}^{49}$Ca, and one-valence-hole ${}^{47}$Ca, nuclei. The isotopes of interest were populated via the cold-neutron capture reactions ${}^{40}$Ca(n,$\gamma$), ${}^{48}$Ca(n,$\gamma$) and ${}^{46}$Ca(n,$\gamma$), respectively. The experiments were performed at the Institut Laue-Langevin, within the EXILL campaign, which employed a large array of HPGe detectors. The $\gamma$ decay and level schemes of these nuclei were investigated by $\gamma$-ray coincidence relationships, leading to the identification of 41, 10, and 6 new transitions in ${}^{41}$Ca, ${}^{47}$Ca, and ${}^{49}$Ca, respectively. Branching ratios and intensities were extracted for the $\gamma$ decay from each state, and $\gamma$-ray angular correlations were performed to establish a number of transition multipolarities and mixing ratios, thus helping in the spin assignment of the states. The experimental findings are discussed along with microscopic, self-consistent beyond-mean-field calculations performed with the Hybrid Configuration Mixing model, based on a Skyrme SkX Hamiltonian. The latter suggests that a fraction of the low-spin states of the ${}^{41}$Ca, ${}^{49}$Ca, and ${}^{47}$Ca nuclei is characterized by the coexistence of either 2p-1h and 1p-2h excitations, or couplings between single-particle/hole degrees of freedom and collective vibrations (phonons) of the doubly-magic "core".

   nucl-exPRC(2021)·11 citations
3. 03*

   Accessing tens-to-hundreds femtoseconds nuclear state lifetimes with low-energy binary heavy-ion reactions

   M. Ciemala🇵🇱 · S. Ziliani🇮🇹 · F. C. L. Crespi🇮🇹 · S. Leoni🇮🇹 · B. Fornal🇵🇱 · A. Maj🇵🇱 · P. Bednarczyk🇵🇱 · G. Benzoni🇮🇹 · A. Bracco🇮🇹 · C. Boiano🇮🇹 · S. Bottoni🇮🇹 · S. Brambilla🇮🇹 and 43 other authors

   A novel Monte Carlo technique has been developed to determine lifetimes of excited states in the tens-to-hundreds femtoseconds range. The method is applied to low-energy heavy-ion binary reactions populating nuclei with complex velocity distributions. Its relevance is demonstrated in connection with the ${}^{18}$O(7.0 MeV/u) + ${}^{181}$Ta experiment, performed at GANIL with the AGATA+VAMOS+PARIS setup, to study neutron-rich O, C, N, ... nuclei. Excited states in ${}^{17}$O and ${}^{19}$O, with known lifetimes, are used to validate the method over the $\sim$20-400 fs lifetime-sensitivity range. Emphasis is given to the unprecedented position resolution provided by $\gamma$-tracking arrays, which turns out to be essential for reaching the required accuracy in Doppler-shift correction, at the basis of the detailed analysis of $\gamma$-ray lineshape and resulting state lifetime determination. The technique is anticipated to be an important tool for lifetime investigations in exotic neutron-rich nuclei, produced with intense ISOL-type beams.

   nucl-exEPJA(2021)·11 citations
4. 04*

   Deep Learning Analysis of Deeply Virtual Exclusive Photoproduction

   Jake Grigsby🇺🇸 · Brandon Kriesten🇺🇸 · Joshua Hoskins🇺🇸 · Simonetta Liuti🇺🇸 · Peter Alonzi🇺🇸 · Matthias Burkardt🇺🇸

   We present a Machine Learning based approach to the cross section and asymmetries for deeply virtual Compton scattering from an unpolarized proton target using both an unpolarized and polarized electron beam. Machine learning methods are needed to study and eventually interpret the outcome of deeply virtual exclusive experiments since these reactions are characterized by a complex final state with a larger number of kinematic variables and observables, exponentially increasing the difficulty of quantitative analyses. Our deep neural network (FemtoNet) uncovers emergent features in the data and learns an accurate approximation of the cross section that outperforms standard baselines. FemtoNet reveals that the predictions in the unpolarized case systematically show a smaller relative median error than the polarized that can be ascribed to the presence of the Bethe Heitler process. It also suggests that the $t$ dependence can be more easily extrapolated than for the other variables, namely the skewness, $\xi$ and four-momentum transfer, $Q^2$. Our approach is fully scalable and will be capable of handling larger data sets as they are released from future experiments.

   ↳ hep-phnucl-exphysics.comp-phphysics.data-anPRD(2021)·30 citations
5. 05*

   Role of exact treatment of thermal pairing in radiative strength functions of ${}^{161-163}$Dy nuclei

   L. Tan Phuc · N. Quang Hung · N. Dinh Dang · L. T. Quynh Huong · N. Ngoc Anh · N. Ngoc Duy · L. Ngoc Uyen · N. Nhu Le🇻🇳

   The enhancement of radiative strength function (RSF) in the region of low $\gamma$-rays energy ($E_{\gamma }\le 12$ MeV), which is caused by the pygmy dipole resonance (PDR), is treated within the phonon damping model (PDM) plus exact thermal pairing (EP) without adding any extra PDR strength function. The numerical calculations performed for ${}^{161-163}$Dy show that, because of the effect of EP, the EP+PDM can describe reasonably well the total RSF data in both low- and high-energy regions of $\gamma$-rays. Consequently, as compared to the conventional description within the phenomenological generalized Lorentzian (GLO) and standard Lorentzian (SLO) models, the EP+PDM calculations can eliminate at least eight free parameters. This indicates the important role of microscopic approaches towards the precise description of the RSF. In particular, temperature is found to have significant contributions to the RSF below the neutron separation energy, questioning again the validity of the Brink-Axel hypothesis in this energy region.

   ↳ nucl-thnucl-exPRC(2020)·5 citations
6. 06*

   Particle-in-Cell Techniques for the Study of Space Charge Effects in the Advanced Cryogenic Gas Stopper

   R. Ringle🇺🇸 · G. Bollen🇺🇸 · K. Lund🇺🇸 · C. Nicoloff🇺🇸 · S. Schwarz🇺🇸 · C. S. Sumithrarachchi🇺🇸 · A. C. C. Villari🇺🇸

   Linear gas stoppers are widely used to convert high-energy, rare-isotope beams and reaction products into low-energy beams with small transverse emittance and energy spread. Stopping of the high-energy ions is achieved through interaction with a buffer gas, typically helium, generating large quantities of He${}^{+}$/e${}^{-}$ pairs. The Advanced Cryogenic Gas Stopper (ACGS) was designed for fast, efficient stopping and extraction of high-intensity, rare-isotope beams. As part of the design process, a comprehensive particle-in-cell code was developed to optimize the transport and extraction of rare isotopes from the ACGS in the presence of space charge, including He${}^{+}$/e${}^{-}$ dynamics, buffer gas interactions including gas flow, RF carpets, and ion extraction through a nozzle or orifice. Details of the simulations are presented together with comparison to experiment when available.

   ↳ physics.acc-phnucl-exNucl.Instrum.Meth.B(2021)·5 citations
7. 07*

   Jets and elliptic flow correlations at low and high transverse momenta in ultrarelativistic A+A collisions

   L.V. Bravina🇳🇴 · G.Kh. Eyyubova🇷🇺 · V.L. Korotkikh🇷🇺 · I.P. Lokhtin🇷🇺 · S.V. Petrushanko🇷🇺 · A.M. Snigirev🇷🇺 · E.E. Zabrodin🇳🇴

   Data from the Large Hadron Collider on elliptic flow correlations at low and high $p_T$ from Pb+Pb collisions at $\sqrt{s_{NN}}=5.02$~TeV are analyzed and interpreted in the framework of the HYDJET++ model. This model allows us to describe simultaneously the region of both low and high transverse momenta and, therefore, to reproduce the experimentally observed nontrivial centrality dependence of elliptic flow correlations. The origin of the correlations between low and high-$p_T$ flow components in peripheral lead-lead collisions is traced to correlations of particles in jets.

   ↳ nucl-thhep-phnucl-exPRC(2021)·11 citations
8. 08*

   Determination of matter radius and neutron skin of ${}^{58}$Ni from reaction cross section of proton+${}^{58}$Ni scattering based on chiral $g$-matrix model

   Shingo Tagami🇯🇵 · Maya Takechi🇩🇪 · Jun Matsui · Tomotsugu Wakasa🇯🇵 · Masanobu Yahiro🇯🇵

   Background: Using the chiral (Kyushu) $g$-matrix folding model with the densities calculated with Gogny-HFB (GHFB) with the angular momentum projection (AMP), we determined the central values of matter radius and neutron skin from the central values of reaction cross sections ${\sigma }_{\mathrm{R}}(\mathrm{EXP})$ of p+${}^{40,48}$Ca and p+${}^{208}$Pb scattering. As for p+${}^{58}$Ni scattering, ${\sigma }_{\mathrm{R}}(\mathrm{EXP})$ are available as a function of incident energy $E_{\mathrm{in}}$. Aim: Our aim is to determine matter radius $r_m$ and skin $r_{\mathrm{skin}}$ for ${}^{58}$Ni from the ${\sigma }_{\mathrm{R}}(\mathrm{EXP})$ of p+${}^{58}$Ni scattering by using the Kyushu $g$-matrix folding model with the GHFB+AMP densities. Results: For p+${}^{58}$Ni scattering, the Kyushu $g$-matrix folding model with the GHFB+AMP densities reproduces ${\sigma }_{\mathrm{R}}(\mathrm{EXP})$ in $8.8\le E_{\mathrm{in}}\le 81$MeV. For $E_{\mathrm{in}}=81$MeV, we define the factor $F$ as $F={\sigma }_{\mathrm{R}}(\mathrm{EXP})/{\sigma }_{\mathrm{R}}(\mathrm{AMP})=0.9775$. The $F{\sigma }_{\mathrm{R}}(\mathrm{AMP})$ be much the same as the center values of ${\sigma }_{\mathrm{R}}(\mathrm{EXP})$ in $8.8\le E_{\mathrm{in}}\le 81$MeV. We then determine $r_{\mathrm{m}}(\mathrm{EXP})$ from the center values of ${\sigma }_{\mathrm{R}}(\mathrm{EXP})$, using ${\sigma }_{\mathrm{R}}(\mathrm{EXP})=C{r}_m^2(\mathrm{EXP})$ with $C=r_m^2(\mathrm{AMP})/(F{\sigma }_{\mathrm{R}}(\mathrm{AMP}))$. The $r_m(\mathrm{EXP})$ thus obtained are averaged over $E_{\mathrm{in}}$. The averaged value is $r_m(\mathrm{EXP})=3.697$fm. Eventually, we obtain $r_{\mathrm{skin}}(\mathrm{EXP})=0.023$fm from $r_{\mathrm{m}}=3.697$fm and $r_p(\mathrm{EXP})=3.685$fm of electron scattering.

   ↳ nucl-thnucl-ex0 citations

* Reconstructed cohort: no mailing for this day survives in the archive. Papers are grouped by their submission times and arXiv's announcement cut-off, assuming announcement without delay; positions follow identifier order. Validated at ~91% exact-day agreement against the archived era.